Relief valves are used in fluid systems to control the internal pressure or vacuum within the system. Pressure relief valves are used to control or limit the amount of pressure in a system by allowing pressurized fluid to flow from an auxiliary passage out of the system. Vacuum relief valves are used to open at a predetermined pressure to admit gas into the system to control the amount of vacuum in a system.
A typical vacuum or pressure relief valve is a direct acting valve. For example, a standard vacuum relief valve design includes a pallet which is forced upwards when pressure in the container drops. The movement of the pallet opens a vent which allows atmospheric gas to enter the container to equalize the pressure difference between atmosphere and container. The movement of the pallet is directly related to the pressure differential between the container and atmosphere.